Purified proteins are produced in systems for a wide range of applications in biology and biotechnology. These include research into cellular and molecular function, production of proteins as biopharmaceuticals or diagnostic reagents, and modification of the traits or phenotypes of livestock and crops.
Affinity protein tags are used as tools for purifying proteins from crude extracts. Protein tags are peptide sequences genetically grafted onto a recombinant protein. Often these tags are removable by chemical agents or by enzymatic means, such as proteolysis or intein splicing. Tags are attached to proteins for various purposes. Elutable affinity tags to purify proteins can be utilized in diverse cell systems including in Escherichia coli, yeast, Drosophila, and HeLa extracts.
Affinity tags are appended to proteins so that they can be purified from their crude biological source using an affinity technique. These include chitin binding protein (CBP), maltose binding protein (MBP), and glutathione-s-transferase (GST). The poly(His) tag is the most widely-used protein tag and it binds to metal matrices.
Solubilization tags are used, especially for recombinant proteins expressed in chaperone-deficient species such as E. coli, to assist in the proper folding in proteins and keep them from precipitating. These include thioredoxin (TRX) and poly(NANP). Some affinity tags have a dual role as a solubilization agent, such as MBP, and GST.
Chromatography tags are used to alter chromatographic properties of the protein to afford different resolution across a particular separation technique. Often, these consist of polyanionic amino acids, such as FLAG tag.
Epitope tags are short peptide sequences which are chosen because high-affinity antibodies can be reliably produced in many different species. These are usually derived from viral genes, which explain their high immunoreactivity. Epitope tags include V5-tag, c-myc-tag, and HA-tag. These tags are particularly useful for western blotting and immunoprecipitation experiments, although they also find use in antibody purification.
Fluorescence tags are used to give visual readout on a protein. GFP and its variants are the most commonly used fluorescence tags. More advanced applications of GFP include using it as a folding reporter (fluorescent if folded, colorless if not).
Protein tags find many other usages, such as specific enzymatic modification (such as biotin ligase tags) and chemical modification (FLaSH) tag. Often tags are combined to produce multifunctional modifications of the protein. However, with the addition of each tag comes the risk that the native function of the protein may be abolished or compromised by interactions with the tag.
For pharmaceutical production, tagged proteins are undesirable and require an extra step of removal of the tags after the purification step. The expression and production of the protein product with the tag can cause conformation changes in the protein resulting in diminished activity, even after the removal of the tag. Additionally, it is difficult to completely remove the tagged protein, leading to the presence of residual impurities in the protein sample. Thus, there is a need for expression and purification systems for producing heterologous proteins without relying on affinity tags for purification. Additionally, there is a need for economical and efficient methods for producing heterologous proteins in bacterial systems.